Maximum, minimisation problem. Don't know where to start.

Hi all,

I have the following problem:

A researcher has funds to buy enough computing power to number-crunch a problem in 8 years. Computing power per dollar doubles every 23 months.
When should he buy his computers to have finished the problem as soon as possible? Give your answer as a decimal in months, accurate within 0.1 months.

Suppose the problem would take C months on current computers. What is the largest value of C for which he should buy the computers immediately? Give an answer as a decimal accurate to 0.1 months.